The present invention relates generally to cardiac pacemakers, and other types of implantable medical devices, which provide electrical stimuli to the heart to control the heart""s rhythm. More particularly, the invention relates to isolating the respiration signal from the heart signal without the use of filters.
Cardiac pacing involves the electrical stimulation of the heart in order to control the timing of the contractions of the heart. Electrical stimuli in the form of pulses are generated by a battery-powered pacemaker and applied to the tissue of the heart by one or more electrodes that are connected to the pacemaker via flexible, insulated conductors. The insulated conductors and associated electrodes form what is referred to as the xe2x80x9clead.xe2x80x9d
Some pacemakers deliver cardiac stimulating pulses at a fixed, programmed rate regardless of the patient""s activity level. Although this technique may be adequate for some, other patients respond better if the pacing system adapts the delivery of the cardiac stimulating pulses based upon the patient""s metabolic demand. In these systems, as metabolic demand increases, the rate of cardiac pacing also increases. These xe2x80x9crate adaptivexe2x80x9d pacing systems need to be able to detect when metabolic demand is changing.
A respiratory-related parameter, which has been found to closely correlate with metabolic demand, is minute ventilation. Minute ventilation may be defined as the volume of air inspired and expired by the patient during a predetermined time period. It has been found that minute ventilation tracks very well with metabolic need over a range of heart rates, and, therefore, can provide a good index for a rate adaptive pacemaker. Specifically, as the patient breathes faster, the patient""s heart rate should pick up, as the faster breathing is often indicative of greater physical activity. However, the techniques used to derive the minute ventilation signal in prior art devices are often contaminated by events other than respiratory activity.
Minute ventilation can be calculated from a measured transthoracic impedance. For example, the transthoracic impedance is often measured between an intracardiac lead implanted in the heart and the case electrode that is implanted in the patient""s body, e.g., under the patient""s pectoral muscle. Measuring the transthoracic impedance using the pacing leads and the case electrode usually presents an undesired heart component in the minute ventilation signal. In particular, the beating of the heart results in a component of the transthoracic impedance being related to the motion of the heart rather than the motion of lungs as a result of the patient inhaling and exhaling. This component of the minute ventilation parameter may be significant enough to result in the pacemaker processor erroneously concluding that the minute ventilation signal is indicative of a false activity level for the patient.
Hence, to use the minute ventilation signal, the heart component generally must be removed. Currently, the heart component is removed by the use of a sharp filter. Hence, use of a filter adds an additional step to the processing of the minute ventilation signal and consumes additional space in the pacemaker case. Moreover, the filter also results in additional consumption of power from the battery that decreases the long-term life of the implanted device.
The minute ventilation signal can also be obtained by periodically sampling the transthoracic impedance to develop the overall minute ventilation signal. To obtain a properly sampled signal, current pacemakers sample at rates between 16-20 samples per second. The high sampling rate ensures that the minute ventilation waveform is accurately reconstructed over all expected frequencies. Because every sample taken consumes battery power, a high sampling rate of 16-20 samples per second can significantly decrease the battery life of the pacemaker.
Hence, a need exists to obtain the minute ventilation signal using the pacing leads and case, but to avoid the use of a filter to remove the heart signal. Also, it is desirable to decrease the sampling rate to obtain the minute ventilation signal to conserve power from the pacemaker battery, effectively increasing the useable life of the pacemaker. Preferably, the minute ventilation signal should be obtained using only a small amount of power.
The present invention provides a unique method of isolating the minute ventilation signal from the heart signal using a low sampling rate and without the use of a filter. On average, the heart rate is approximately four times the respiratory rate. For example, a person""s heart may beat, on average, sixty times per minute while the same person breathes only sixteen times per minute. The present invention samples the transthoracic impedance, which is indicative of a minute ventilation signal without obtaining a heart signal component by synchronizing the sampling rate with the heart rate.
One technique for sampling the impedance without obtaining a heart component is to provide a sampling rate, which samples the impedance at the zero crossings or during the quiescent periods of the heart signal. Because the heart rate is approximately four times faster than the respiratory rate, sampling at the zero crossing of the heart rate provides adequate data to determine the minute ventilation signal while reducing contributions from the heart signal.
In this embodiment, because the sampling occurs at the zero crossings of the heart signal, there are no significant heart components sampled. Therefore, the majority of the signal sampled is a result of the patient""s respiration. By synchronizing the sampling rate with the heart rate, sampling can be selected to only occur when the sampled signal is primarily indicative of respiration and this signal can then be used to produce a minute ventilation parameter. Therefore, the need to filter out the heart signal is eliminated.
In another embodiment, the sampling rate is selected so as to be synchronized with the heart rate such that the transthoracic impedance is sampled at least once each heart cycle at the same interval during the heart signal. If the transthoracic impedance is sampled at the same interval during each heart cycle, the heart""s contribution to the impedance signal can be ignored as a constant. The contribution to the transthoracic impedance due to the patient""s respiration can then be reconstructed so as to provide the minute ventilation parameter.
The present invention also conserves power by decreasing the sampling rate. Because the sampling rate is synchronized such that the heart signal component can be ignored, a sampling rate of approximately twice the heart rate will obtain adequate data. As an average heart rate is 60 beats per minute, or one beat per second, the average sampling rate under the present invention is between 2-4 samples per second. The significant decrease from the 16-20 samples per second used in filter based systems conserves battery power of the pacemaker.
One embodiment of the present invention is a method of sampling a respiratory signal by a pacing system. The method includes the steps of determining a heart signal having a heart rate and establishing a sampling rate at approximately about twice the heart rate. The sampling rate is then synchronized with the heart signal and the respiratory signal sampled at the synchronized sampling rate. This method results in the respiratory signal data being sampled at about the zero-crossings of the heart signal or being sampled at an interval of the heart signal such that the heart signal contribution can be ignored.
Another embodiment of the present invention is a method of determining minute ventilation data by a pacing system. The method includes the steps of detecting the heart signal and sampling the respiratory signal and the heart signal at a sampling rate. The sampling rate is synchronized with the heart rate so the samples are obtained at about approximately the zero crossings of the heart signal, thereby removing the heart component from the respiratory signal without the use of a filter. Alternatively, the sampling rate is selected so that the heart component is essentially a constant for each measurement and can, thus, be ignored. The minute ventilation data is then determined from the respiratory signal. The minute ventilation data is determined by calculating the amplitude times the frequency of the respiration signal.